My son entered kindergarten at 4 and had a lot of issues with social/emotional maturity. I didn't push him ahead -- he made the cutoff with a week to spare. Even though he was already academically advanced for his age, I seriously considered holding him back a year for the emotional maturity but since he had already taught himself to read, I decided holding him back would be an academic disaster.
He had an amazing kindergarten teacher who was able to challenge him and gave him work on his level, while dealing with other kids who barely knew their letters. Socially and emotionally he struggled and had morning meltdowns frequently. His teacher often had to peel him out of the car for me. He didn't really make friends in his class.
I'm glad now that I didn't hold him back, because 1st grade was a total nightmare academically, although it was a great success socially. He was so bored and unchallenged in 1st grade, that I couldn't imagine where we'd have been if I'd held him back. I really learned what a difference the right teacher makes, since he went from an amazing kinder teacher to a terrible first grade one. He had a great year socially however, and made good friends and wanted play dates with school friends.
I guess my point is that the success of early kinder will depend on how mature your son is. If maturity is an issue you might consider starting kinder on time and skipping a grade later when he is more mature. The difference in maturity between an 8.5yo and a 9.5yo is much smaller than the difference between a 4.5yo and 5.5yo.
